For a display which becomes a core device of a digital network with the approach of the multimedia age, there is demanded a display which is made to have a high resolution, a thinner thickness, a large screen, and the like. As a display, devices using a cathode lay tube (CRT) have been used extensively, but a thin and high resolution self-luminous type display is now demanded because the CRT increases its installing space and weight as the CRT is made large (to have a larger screen).
In response to such a demand, a plasma display panel (PDP) has been put to practical use. The PDP can display various types of information finely with a high resolution and has characteristics that it can be produced to have a large and thin screen. But, the PDP does not necessarily have sufficient performance as compared with the CRT in view of brightness, contrast, color reproducibility, power consumption and the like.
Meanwhile, a display using electron emission elements such as a field-emission cold cathode, which is a so-called field-emission display (FED), has the same basic display principle as that of the CRT and similar performance as that of the CRT. Specifically, the FED is receiving attention as a thin image display or the like because it has characteristics such as a wide viewing angle, a quick response speed and a small power consumption in addition to the basic display performance such as brightness, contrast and color reproducibility.
The FED has a rear plate, which has many electron emission type electron emission elements formed as electron sources on a substrate, and a face plate which is formed of a glass substrate or the like on which a phosphor layer is formed. The rear plate and the face plate are disposed to face each other with a small gap between them. The gap between these plates is sealed airtight in a vacuum state.
Incidentally, the phosphor layer for the FED has not been necessarily studied sufficiently. Under present circumstances, phosphors for emission of blue light, green light and red light are selectively used based on experiences from the phosphors which have been used for the CRT. For example, for a blue light-emitting phosphor and a green light-emitting phosphor, it is being considered using a ZnS:Ag phosphor and a ZnS:Cu, Al phosphor having a crystal structure of a cubic system for the CRT excelling in emission luminance. It is also considered using a Y2O2S:Eu phosphor or the like for the red light-emitting phosphor.
However, simple diversion of the phosphors for emission of blue, green and red light of the CRT to the FED causes problems that the phosphor layer gets burn-in (deterioration of the phosphors) incident to the display operation of the FED and emission characteristics deteriorate with time. Degradation of the phosphors with time is particularly prominent in the blue light-emitting phosphor and the green light-emitting phosphor. When luminance of the phosphors for the FED degrades with time, the emission colors of the FED change, resulting in generation of a color drift.
The cause of the above-described degradation of the characteristics of phosphors for the FED is not fully clarified yet, but it is considered being caused by a fact that an acceleration voltage (excitation voltage) of the electron beam for making the phosphor layer emit light is lower than that for the CRT. Specifically, the CRT has an acceleration voltage of 25 kV to 30 kV, while the FED has a low acceleration voltage of approximately 3 kV to 15 kV for the electron beam.
As described in, for example, Japanese Patent Laid-Open Application Nos. 2-255791 JP-A and 11-349937 JP-A, it is proposed to use a phosphor, which has 50% or more of a crystal structure formed of ZnS:Ag, Al or ZnS:Ag, M, Al (M is at least one selected from Au and Cu, and a content of M is in a range of 0.1 to 10 ppm) of a hexagonal system, as a blue light-emitting phosphor for the CRT.
But, because such a blue light-emitting phosphor is based on the premise that it is used for a large CRT or the like, a general high voltage (27 kV in the above publications) is used as the acceleration voltage for the electron beam. Thus, it is described that the ZnS phosphor of the hexagonal system is effective in improving the current characteristic of a high current area side, but there is nothing described about a case that the electron beam has a low acceleration voltage of 3 kV to 15 kV. In other words, it is not considered at all that the ZnS phosphor of the hexagonal system described in the above publications is used for the FED.
Japanese Patent Laid-Open Application No. 62-95378 JP-A describes that the ZnS phosphor having a content of a hexagonal system in a range of 0.5 to 12% is used as a blue light-emitting phosphor for the CRT. This ZnS phosphor just has an improved current characteristic of the CRT that the acceleration voltage for the electron beam is high, and there is no disclosure about a case that the electron beam has a low acceleration voltage of 3 kV to 15 kV. The ZnS phosphor, which contains the hexagonal system at a prescribed ratio, is not considered to be used for the FED at all. The above-described publications indicate that when a content of the hexagonal system exceeds 12% and the acceleration voltage is 20 kV, emission intensity lowers.
It is an object of the present invention to provide a phosphor for a display which enables to keep good emission luminance for a long time when it is used as a phosphor or the like for the FED, while suppressing degradation of a characteristic (luminance degradation) with time which may result from a low acceleration voltage of an electron beam. It is also an object to provide a field-emission display which has improved display characteristics and reliability and a life characteristic also improved by using such a phosphor for a display.